Fuel cells have been proposed for many applications including electrical vehicular power plants to replace internal combustion engines. Hydrogen is often used as the fuel and is supplied to the fuel cell's anode. Oxygen (as air) is the cell's oxidant and is supplied to the cell's cathode.
The hydrogen used in the fuel cell can be derived from the reformation of methanol or other organics (e.g., hydrocarbons). Unfortunately, the reformate exiting the reformer contains undesirably high concentrations of carbon monoxide which can quickly poison the catalyst of the fuel cell's anode, and accordingly must be removed. For example, in the methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide according to the reaction: EQU CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +3H.sub.2
This reaction is accomplished heterogeneously within a chemical reactor that provides the necessary thermal energy throughout a catalyst mass and actually yields a reformate gas comprising hydrogen, carbon dioxide, carbon monoxide, and water. One such reformer is described in U.S. Pat. No. 4,650,727 to Vanderborgh. Carbon monoxide (i.e., about 1-3 mole %) is contained in the H.sub.2 -rich reformate/effluent exiting the reformer, and must be removed or reduced to very low nontoxic concentrations (i.e., less than about 20 ppm) to avoid poisoning of the anode.
It is known that the carbon monoxide, CO, level of the reformate/effluent exiting a methanol reformer can be reduced by utilizing a so-called "shift" reaction. In the shift reactor, water (i.e. steam) is added to the methanol reformate/effluent exiting the reformer, in the presence of a suitable catalyst, to lower its temperature, and increase the steam to carbon ratio therein. The higher steam to carbon ratio serves to lower the carbon monoxide content of the reformate according to the following ideal shift reaction: EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2
Some CO still survives the shift reaction. Depending upon the reformate flow rate and the steam injection rate, the carbon monoxide content of the gas exiting the shift reactor can be as low as 0.5 mole %. Any residual methanol is converted to carbon dioxide and hydrogen in the shift reactor. Hence, shift reactor effluent comprises hydrogen, carbon dioxide, water and some carbon monoxide.
The shift reaction is not enough to reduce the CO content of the reformate enough (i.e., to below about 20 ppm). Therefore, it is necessary to further remove carbon monoxide from the hydrogen-rich reformate stream exiting the shift reactor, and prior to supplying it the fuel cell. It is known to further reduce the CO content of H.sub.2 -rich reformate exiting the shift reactor by a so-called "PROX" (i.e., preferential oxidation) reaction effected in a suitable PROX reactor. The PROX reactor comprises a catalyst bed operated at temperatures which promote the preferential oxidation of the CO by air in the presence of the H.sub.2, but without consuming/oxidizing substantial quantities of the H.sub.2. The PROX reaction is as follows: EQU CO+1/20.sub.2 .fwdarw.CO.sub.2
Desirably, the O.sub.2 required for the PROX reaction will be about two times the stoichiometric amount required to react the CO in the reformate. If the amount of O.sub.2 exceeds about two times the stoichiometric amount needed, excessive consumption of H.sub.2 results. On the other hand, if the amount of O.sub.2 is substantially less than about two times the stoichiometric amount needed, insufficient CO oxidation will occur. The PROX process is described in a paper entitled "Methanol Fuel Processing for Low Temperature Fuel Cells" published in the Program and Abstracts of the 1988 Fuel Cell Seminar, October 23-26, 1988, Long Beach, Calif., and in U.S. Pat. Vanderborgh et al 5,271,916, inter alia.
PROX reactions may be either (1) adiabatic (i.e. where the temperature of the catalyst is allowed to rise during oxidation of the CO), or (2) isothermal (i.e. where the temperature of the catalyst is maintained substantially constant during oxidation of the CO). The adiabatic PROX process is typically effected via a number of sequential stages which progressively reduce the CO content in stages. Temperature control is important in adiabatic systems, because if the temperature rises too much, a reverse shift reaction can occur which actually produces more CO. The isothermal process can effect the same CO reduction as the adiabatic process, but in fewer stages (e.g., one or two stages) and without concern for the reverse shift reaction.
In either case (i.e., adiabatic or isothermal), a controlled amount of O.sub.2 (i.e., as air), is mixed with the reformate exiting the shift reactor, and the mixture passed through a suitable catalyst bed known to those skilled in the art. To control the air input, the CO concentration in the gas exiting the shift reactor is measured, and based thereon, the O.sub.2 concentration needed for the PROX reaction adjusted. However, effective real time CO sensors are not available, and accordingly system response to CO concentration measurements is slow. Alternatively for adiabatic systems, the catalyst temperature can be used as a reference to control the O.sub.2 feed rate. Catalyst temperature cannot be used to control O.sub.2 feed to an isothermal PROX reactor.
For the PROX process to be most efficient in a dynamic system (i.e., where the flow rate and CO content of the H.sub.2 -rich reformate vary continuously in response to variations in the power demands on the fuel cell system), the amount of O.sub.2 (i.e., air) supplied to the PROX reactor must also vary on a real time basis, in order to continuously maintain the desired oxygen-to-carbon monoxide concentration ratio.